1. Field of the Invention
More stringent environmental regulations have forced manufacturers of elemental phosphorus to seek alternate methods for dealing with the phosphorus sludge problem. Accordingly, past practices which included the storage of sludge in settling ponds and sumps may no longer be considered viable. Likewise, storage in metal tanks can only be considered as a temporary measure. Although elemental phosphorus is only very slightly soluble in water and accordingly is considered to leach at a very slow rate, efforts need to be made to prevent its entrance into ground water and aquifers.
Although elemental phosphorus itself is pyrophoric and highly toxic, the more oxidized inorganic salts of phosphorus are generally much less hazardous. In fact, phosphorus in the form of orthophosphate (PO.sub.4) is required for all earthly life forms and is employed in huge quantities as fertilizer and animal food supplement. The present invention provides a technically efficient and economically viable means for the conversion of such elemental phosphorus containing sludges into certain feedstock eminently suitable for later preparation of plant food material.
2. Description of the Prior Art
The conversion of elemental phosphorus containing waste-waters and sludges into fertilizer products has been proposed by Barber et al. (James C. Barber, Charles B. Hendrix, and David Mussleman, Chemtech, May, 1986, pp. 298-302). U.S. Pat. No. 4,383,847, Barber, May 17, 1983, teaches that clarified and centrifuged waste-water from phosphorus condensers be used in conjunction with phosphoric acid and ammonia for the production of suspension fertilizers. However, Barber's teachings include no apparent means to ensure that the elemental phosphorus in his wastewater (stated to be about 12 ppm P.sub.4) is oxidized to soluble phosphate before incorporation into such suspension fertilizers. In U.S. Pat. Nos. 4,451,277, Barber, May 29, 1984; 4,537,615, Barber, Aug. 27, 1985; and 4,514,366, Barber, Apr. 30, 1985, there is further teaching that phosphorus sludge can be combusted in a graphite chamber to produce phosphoric acid which is then added to the phosphorus plant condenser water. The condenser water is subsequently treated with ammonia and processed to a suspension fertilizer. Again, no provisions are provided or taught for the oxidation of the elemental phosphorus dissolved or suspended in such condenser solution.
The teachings in U.S. Pat. No. 4,686,094, Roberts et al., Aug. 11, 1987, suggest treatment of elemental phosphorus-containing wastes by contacting the material with oxygen-containing gas such as air while maintaining thereover a cover of nonflammable liquid such as water. Upon complete oxidation, the phosphorus is recovered as much less hazardous dilute phosphoric acid. A major drawback and substantial disadvantage of this process is the very long reaction times required, i.e., upwards of 6 months. Also compounding such disadvantage of slow reaction are the attendant requirements of maintaining temperatures during cold-winter months and substantial input of mechanical shear energy to ensure interfacing of the normally very viscous sludge with the air introduced or sparged therewith.
In contrast to the teaching of the present invention, and the processing described above, most prior art processes involve the physical or chemical treatment of phosphorus sludge in order to enhance the recovery of elemental phosphorus therefrom, rather than the total chemical conversion of the phosphorus to a different, useful, by-product.
Examples of physical processes for recovering purified elemental phosphorus from phosphorus sludge may be found in the following: U.S. Pat. Nos. 3,084,029, Apr. 2, 1963; 3,113,839, Dec. 10, 1963; and 3,136,604, Jun. 9, 1964, Barber et al. (assigned to the assignee of the present invention), expound on the use of centrifugation as a means of recovering purified elemental phosphorus from phosphorus bearing sludges and residues. In U.S. Pat. No. 3,104,952, Hartig, Sep. 24, 1963, there is taught the mixing of phosphorus containing sludge with 75 to 95 weight percent aqueous H.sub.3 PO.sub.4 followed by steam distillation in an inert atmosphere to recover purified elemental phosphorus. U.S. Pat. No. 4,481,176, Dodson et al., Nov. 6, 1984, suggest homogenization of the sludge followed by high-pressure filtration through thin cake filters. U.S. Pat. Nos. 4,492,627, Crea, Jan. 8, 1985, and 4,595,492, Crea et al., Jun. 17, 1986, propose the use of hydrocyclones as a means of separating elemental phosphorus from other impurities in phosphorus sludges. U.S. Pat. No. 4,762,697, Nield et al., Aug. 9, 1988, teach the use of distillation in an inert atmosphere followed by an input of air to burn off phosphorus residues and enhance solids recovery.
In a combination physical-chemical process the teachings in U.S. Pat. No. 4,608,241, Barber, Aug. 26, 1986, propose distilling phosphorus-containing waste to thereby separate and recover vaporized phosphorus and water from the non-volatile residues. The resulting non-volatile residues are then agglomerated and smelted in a submerged-arc electric furnace to further liberate phosphorus vapor for recovery. In the event that a phosphorus furnace is not available, the nonvolatile residues are diluted with large amounts of other feedstock and granulated to produce fertilizer products.
In addition to the physical processes described above, a number of chemical treatments have been suggested to enhance the recovery of elemental phosphorus from sludges. In general, the effectiveness of chemicals in these processes is assumed to result from their ability to attack and remove surface films and impurities from elemental phosphorus globules, thereby allowing them to coalesce and otherwise unite to form larger particles which are more amenable to separation and recovery by gravitational settling or other physical means. In general, the chemical is added at concentrations sufficient to promote the separation and removal of phosphorus from the sludge without significant chemical attack on the bulk of the phosphorus contained in the mixture. For example, the teachings in U.S. Pat. Nos. 3,436,184, Hinkebein, Apr. 1, 1969, and 3,442,621, Hinkebein, May 6, 1969, suggest the addition of chromic acid, or for that matter any oxidizing agent, to promote the separation and recovery of elemental phosphorus and thereby reduce the quantity of phosphorus remaining in the sludge. As another example of this chemical approach, see Imai et al. (Jpn. Kokai Tokkyo Koho 79 93,692, Jul. 24, 1974), wherein is proposed treatment of crude phosphorus with HNO.sub.3 --H.sub.2 SO.sub.4 mixtures to allow separation of highly purified elemental phosphorus.
At first sight, both of the above chemical treatment processes may superficially appear similar to the HNO.sub.3 --H.sub.2 SO.sub.4 oxidation method of the present invention. It should be noted, however, that the present invention, which is explained in greater detail infra, involves the use of concentrated HNO.sub.3 --H.sub.2 SO.sub.4 mixtures in certain critical proportions to ensure the complete oxidation of all elemental phosphorus present in the sludge, while the above methods utilize chemicals for the destruction of films and impurities on the surface of phosphorus globules thereby allowing their coalescence and recovery as elemental phosphorus. Thus, these prior art disclosures actually teach away from the gist underlying the instant invention by suggesting the use of an HNO.sub.3 --H.sub.2 SO.sub.4 mixture having a concentration and quantity much lower than the threshold combination found to be critical for practice of the instant invention.